1. Field of the Invention
This invention relates to an aluminum composite material having a low thermal expansion coefficient and high thermal conductivity, and also relates to a method of producing such an aluminum composite material. The aluminum composite material of the present invention is suitable for a heat sinking plate or the like used in a semiconductor device.
2. Related Art
Recently, in the semiconductor field, the capacity of transistors has been much increased, and LSIs have been formed into a highly-integrated, high-speed and high-performance design, and thus the performance of the semiconductor devices has been improved markedly. Therefore, it has now been important to dissipate thermal energy, produced from semiconductor devices, to a heat sinking plate as efficiently as possible. With respect to conventional heat sinking plate materials for semiconductor devices, copper (Cu) has been used for a substrate (or base plate), molybdenum (Mo) for a large-size substrate, a plastics material or alumina (Al.sub.2 O.sub.3) for a package, and aluminum nitride (AlN) for a large-capacity package.
In the conventional heat sinking plate materials, copper, which has high thermal conductivity of 390 W/(m.multidot.K) at normal or ordinary temperature, is excellent in heat-radiating property. However, there is a large difference in thermal expansion coefficient between copper and silicon (Si) used as a semiconductor material for forming a transistor and an LSI chip (Silicon has a thermal expansion coefficient of 4.2.times.10.sup.-6 /K while copper has a thermal expansion coefficient of 17.times.10.sup.-6 /K). Therefore, there is a problem that a solder bonding surface of Pb--Sn or the like, formed between the heat sinking plate and the semiconductor device, is separated by thermal stresses applied repeatedly during the operation of the associated circuit. In contrast, molybdenum has a thermal expansion coefficient of 5.1.times.10.sup.-6 /K close to that of the semiconductor device, and therefore provides an excellent reliability at the solder bonding surface. However, there is another problem that molybdenum does not possess an adequate heat-radiating property since it has low thermal conductivity of 150 W/(m.multidot.K). AlN is a well-balanced ceramics material because its thermal conductivity is 170 W/(m.multidot.K), and its thermal expansion coefficient is 4.5.times.10.sup.-6 /K. However, AlN has still another problem that it is costly, and therefore is disadvantageous from an economical point of view. Besides, each of these conventional materials is composed of a single material, and therefore it is difficult to arbitrarily control the two properties (that is, the thermal expansion coefficient and the thermal conductivity).
On the other hand, Japanese Patent Examined Publication No. 7-26174 discloses an auxiliary electronic part material as a new heat sinking plate material for semiconductors, which material is composed of aluminum or an aluminum alloy and green colour silicon carbide. Japanese Patent Unexamined Publication No. 64-83634 discloses a low-thermal expansion, high-heat radiating aluminum composite material consisting of at least one selected from the group consisting of aluminum nitride, silicon carbide, boron nitride and graphite, and aluminum. However, these heat sinking plate materials do not have sufficient thermal conductivity. For example, thermal conductivity of an aluminum composite material (disclosed in Japanese Patent Unexamined Publication No. 64-83634) containing 60% by volume of silicon carbide has thermal conductivity of 119 W/(m.multidot.K) (see Table 3), and therefore is not sufficient in heat-radiating property. A composite material (disclosed in Japanese Patent Examined Publication No. 7-26174) containing 50% by volume of green colour silicon carbide has thermal conductivity of 170 W/(m.multidot.K) (see Table 1), and if it is desired to obtain thermal conductivity of not less than 150 W/(m.multidot.K), it is impossible to increase the silicon carbide content to not less than 60% by volume. Namely, in the conventional heat sinking plate materials, the range of choice of the silicon carbide content is narrow, and is not sufficient to meet the thermal expansion coefficient of the semiconductor materials.
There is known a method of producing an aluminum composite material containing silicon carbide, in which a porous preform of silicon carbide is forcibly impregnated under pressure with molten aluminum. In this method, however, the size of an apparatus becomes large, and therefore the production cost is high, and a yield rate is low since a riser is required, and much time and labor are required for preventing leakage of the molten material. Particularly when producing a small-size product, it is difficult to effect a net shape compacting, and there is encountered a problem that a crack is liable to develop in the preform because of the use of a high pressure.